Click here to listen to the live police scanner feed from the incident.

There are reports that as many as three men in military-style uniforms are carrying out the shootings.

Several law enforcement agencies converged on the scene in San Bernardino and were searching for the suspects. Triage units were being set up in the area.

Some people were seen being wheeled away on trolleys after gunfire erupted at the Inland Regional Centre. Others walked quickly from a building with their hands in the air and were searched by officers before being reunited with loved ones.

Police were working to secure the scene about 60 miles east of Los Angeles, Sgt Vicky Cervantes said.

There are reports of 15 to 20 victims in this shooting, via @SBCityFire.

Lieutenant Rich Lawhead said today there are reports of multiple victims.

No arrests have been made and Sergeant Vicky Cervantes says police are working to secure the scene in the inland region east of Los Angeles.

The San Bernardino Police Department said in a tweet it "confirmed an active shooter in the area of Orange Show Rd/ Waterman Ave near Park Center."

The San Bernardino Sheriff urged residents to avoid the area.

A spokeswoman from nearby Loma Linda Medical Center said the hospital was expecting patients momentarily. "We're all kind of on standby right now - it could be any minute," spokeswoman Briana Pastorino said.

A petrol station manager near the site said there is a flood of police activity in the area.

Ana Fuentes, manager of a Shell station about a block from the scene, estimated that there are about 150 police officers flooding the area.

She said she has not got any instructions to stay inside but that customers coming in told her about the shootings at Inland Regional Centre.

The centre says on its website that it provides social services to people with developmental disabilities.

Dozens of people filed out of the scene, holding their hands in their air. Other people were seen being wheeled away on trollies.